One of the things that you have to get used to when running Magento is dealing with the indexes that it relies on. Usually you can do this from the admin panel by going to System -> Index Management.
These articles are about rebuilding the indexes from the Magento Admin
When Reindexing in Magento Admin Fails, Use The Command Line
Another failing of Magento is that when a shop gets to a certain size the rebuilding of the indexes cannot be successfully rebuilt from the Magento Admin.
The following article gives all the reason why Re-Indexing fails. The main one is still the size of the shop.
- Catalog Search Index refresh running slow or halting/freezing (This article also described what the Indexes do)
When indexing fails from the admin panel you get a message saying that it failed with no reason why it failed. Now in most cases you cannot ignore this. You may find that most (if not all) of your products has disappeared from the front end of the website.
How To Re-Index from the command line
The following articles show you a variety of articles of rebuilding the index from the command line. Rebuilding the indexes from the command line requires less resources and allow the task to complete successfully. A CRON job is a scheduled command line command.